The present invention relates to a positioning device, an electronic instrument, and a storage medium storing a program.
A global positioning system (GPS) is widely known as a positioning system using satellites, and is utilized for a car navigation system and the like. The GPS allows the present position to be located by acquiring/extracting a GPS satellite signal from signals received from a plurality of GPS satellites, and performing positioning calculations based on a navigation message acquired from the GPS satellite signal and the like.
The GPS satellite signal is a spread-spectrum-modulated signal called a coarse and acquisition (C/A) code. The polarity of the C/A code is reversed every 20 milliseconds (ms) according to the navigation message (e.g. JP-A-11-258326).
In order to extract and acquire the GPS satellite signal from the received RF signal, the reception sensitivity (hereinafter simply called “sensitivity”) is increased by filtering the received signal (or an intermediate-frequency signal converted from the received signal) to remove unnecessary frequency components. In particular, since the GPS satellite signal has a specific frequency, the sensitivity may be increased by narrowing the passband of the filter in order to extract only components around the specific frequency (or the frequency of the GPS satellite signal when converted into an intermediate-frequency signal).
However, even if the GPS satellite signal has a specific frequency when transmitted from the GPS satellite, the GPS satellite signal does not have the specific GPS frequency when the GPS receiver receives the GPS satellite signal due to the Doppler shift caused by the movement of the GPS satellite around the earth, effects occurring when the signal passes through the ionosphere and the atmosphere, effects caused by a multipath, and the like. Therefore, when narrowing the passband of the filter to a large extent, the spread-spectrum-modulated GPS satellite signal may be partially lost, whereby the positioning accuracy (hereinafter simply called “accuracy”) may decrease.
On the other hand, when increasing the passband of the filter, the number of noise components other than the GPS satellite signal increases, whereby the reception sensitivity decreases due to deterioration in S/N ratio. Specifically, since the accuracy and the sensitivity have a trade-off relationship, the filter must be designed so that both accuracy and sensitivity are achieved to a certain extent.